Gelatin sheets, collagen sheets and sponges are a group of biomaterials that have been used extensively in the medical field. They are dry and absorb and retain large amounts of water. This group of biomaterials is both biocompatible and biodegradable causing little or no inflammation. These biomaterials are useful as wound dressings, artificial skin scaffolds and therapeutic drug delivery devices, whereby the biomaterials can retain therapeutics and deliver such therapeutics to appropriate cells and tissues, as exemplified in Applicant's U.S. Pat. No. 6,475,516.
Gelatin and collagen sheets as provided by the prior art are typically inelastic and may have toxic properties. For example, U.S. Pat. No. 3,491,760 describes a pliable, but inelastic, adhesive multilayer wound covering composed of collagen or gelatin which is “foamed” using air bubbles, treated with a plasticizer (glycerol), cross-linked with a 4% glutaraldehyde solution, and the resulting gel film covered with an adhered cover layer. As is generally known collagen and gelatin matrices cross-linked with glutaraldehyde show residual toxicity.
Gelatin sponge materials such as Gelfoam™, are also known. These materials are absorbable, sterile and water insoluble and used to control bleeding during surgery and may also be provided in a powder format. The material is non-elastic. Another non-elastic gelatin sponge currently used is Surgifoam™. Gelfim™ is a non-elastic material obtained from a formaldehyde cross-linked gelatin solution.
Hydrolyzed collagen, also known as gelatin, has been used as a vehicle for the delivery of therapeutics when covalently attached to the surface of tubular medical devices such as catheters and stents as described in Applicant's U.S. Pat. Nos. 6,132,765 and 6,228,393. The material is hydrated and minimally elastic.
Several patents describe cross-linked inelastic collagen materials such as for example U.S. Pat. Nos. 4,703,108, 4,970,298, 5,550,187, 5,744,545, and 6,132,765.
Sehal and Vijay (Anal. Biochem. 218: 87-91, 1994) describe a method for water-soluble carbodiimide-mediated amidation by using 1-ethyl-3-[3-dimethylaminopropyl carbodiimide (EDC)/N-hydroxy succinimide (NHS). The method is known to cross-link collagen, gelatin and other proteins. However, the resultant matrices are not elastic.
Choi et al. (Biomaterials 20: 409-41, 1999) describe the fabrication of a gelatin-containing artificial skin that contains gelatin and alginate and is cross-linked with EDC. A soluble sponge was immersed in a solution of acetone:water (9:1 by volume) containing 20-100 mg EDC and cross-linked for 24 hr. The EDC when dissolved in water is stated to be deactivated and rapidly loses its cross-linking ability and thus the EDC was dissolved in 90% acetone. The matrix material produced is inherently inelastic.
In the pharmaceutical industry, soft elastic gelatin capsules or “Softgels” are made from a gelatin solution that is plasticized with propylene glycol, sorbitol, glycerin or other approved mixtures. However, the gelatin in the softgel capsules is not cross-linked in order that the capsules remain soluble when ingested. Furthermore, softgel capsules lack elasticity and resistance to the action of degradative processes such as increased temperature and mild enzymatic action.
There is a need to produce gelatin matrices that are elastic, and have appreciable tensile strength while not being prone to becoming hard and brittle.